Adding an extra temperature sensor to the Asus A7V Computer Motherboard.
How to substitute one thermister for another with a different (higher) beta.
(And very possibly other boards that use thermistors)
I recently upgraded my Computer which uses a A7V V1.04 motherboard by installing a 1300 Duron and in the process had to consider the the higher temperatures involved.
I noticed a spare pin header (JTPWR) that the manual refers to as suitable for a power supply temperature sensor. The first thing I did was to try to find the specifications. I had no luck but did find a reference that said that Asus chip involved had a similar specification to the Winborne chip and it's specification showed that a Radio Shack thermistor part number 271-0110 was to be used. If you can get this part you can just wire it up to a suitable connector without any extra resistors so you can skip this next section..Data on The 271-0110 was available at http://support.radioshack.com/support_supplies/doc33/33553.pdf
Using available thermistors.
The correct thermistor has a resistance of 10k ohms at 25°C , 4.161k at 50°C and a "B"value of 3435 +- 1%. The B value in simple terms is how much the resistance changes with temperature. I confirmed that this is the correct part for the A7V by inserting a 4.161k resistor where the thermister goes and getting a reading of 50°C.
It was easy to find thermistors with the required 10k ohms at 25°C but getting the correct B value was impossible. All of the thermistors that I found had a B that was too high. However by the addition of 2 resistors they could be made to work.
The first step in using an alternate thermistor is to determine its resistance at 50°C, usually you get this from a data sheet. The correct part would have a value of 4.161k ohm but so long as yours is lower look on the graph below and determine the value of the "RS" resistor. For example a "Curve Matched" 10k thermistor (Unitherm Type EC95) available from RS Components ( stock # 151-237) for about $2 has a resistance of 3.894k ohm @ 50°C and would require an "RS" resistor of .33k ie. 330 ohms. Putting this resistor in series with the thermistor would increase the total resistance and make the readings incorrect but it can be balanced by a resistor that is connected across the thermistor and the RS resistor . This resistor (RP) is calculated as follows:-
RP = 10 + (100/RS) k ohms thus for the above device RP = 10 + (100/.33) k =313.03k ohm
Selecting values from the 1% resistor range you would use RS=330 ohm and RP= 300k ohm. This combination of resistors and a Unitherm EC95 gives a resistance of 4.165k at 50°C and 9.986k at 25°C and is within .2% of the specification of the desired 271-0110 thermister.
thermistor at 25°C
i.e. 10k there are combinations of RS and RP that maintain the
overall resistance of the circuit at 10k. For example with RS=0 (short
circuit) RP must be infinite (open circuit)
to maintain 10k. In this condition all of any change in the resistance
of the thermistor is reflected in the overall resistance of the
circuit. With RS=infinite (open circuit)
RP must be 10k and in this condition none of any change in the
resistance of the thermistor is reflected in the overall resistance of
There are intermediate values of RS and RP where the overall
resistance of the circuit is still 10k at 25°C but less than the full
amount of any change in the resistance of the thermistor is reflected
the overall resistance of the circuit. Thus a thermistor with a larger
change can be substituted for one with lesser change by adding a couple
Putting it together.
If you have the correct thermistor solder it across the wires going to the connector.
Otherwise work out the two resistor values you need and then select the closest values from the 1/4 Watt 1% range.
You will need a connector for the board. I used an old fan connector, it was 3 pin but It fitted on the 2 pin header, I just did not use the extra pin hole. The Power Supply Thermal Sensor Connector (2-pin block marked JTPWR) is located at the top edge of the board near the RAM slots (see manual page 40).
Arrange the two resistors and the thermistor so that they can be soldered and insulated with heat-shrink tubing without shorting.
Resistor RS connected to thermistor.
First lead connected, polarity is not important.
Resistor RP connected from thermistor to red wire and black lead added.
Note that the sleeve should not be able to slide down the thermistor leg. To finish off cover the lot with another heat-shrink sleeve.
To see the temperature using the Mother Board Monitor program select Asus 3 in the temperature menu. Asus Probe software seems not to have the ability to monitor this Asus feature.
You MUST NOT use this temperature probe INSIDE THE POWER SUPPLY as it is DANGEROUS and also electrical noise may be introduced into the computer, also do not use it on the outside of the computer case. I used mine for room temperature by mounting it near a air inlet to the case. This has an advantage over a normal thermometer in that the temperature can be logged to a file by MBM.
When installing the probe make sure that the computer power lead is disconnected from the wall socket and take precautions against static electricity.Note this method of substituting one thermistor for another may have general application. You just need a thermistor with the same resistance at 25°C and a higher beta. Unfortunately the mathematics to provide a general formula for any required thermister is beyond me. The table above was derived from experiment.
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